The output signal from sensor elements in a sensor, such as a focal plane array, IR FPA, contained in a camera can vary quite considerably as a function of the irradiated effect. Sensor elements thus require to be calibrated with one another. The sensor elements contained in a sensor in an IR camera do not behave in the same way, for example, but exhibit variations in gain and offset. In order to manage these variations, so-called gain maps and offset maps are recorded and stored in production. With the help of the gain map, corrections are made during operation for variations in gain in the individual sensor elements in a sensor. The offset map is used correspondingly to shift the sensor signals of the constituent sensor elements in parallel during operation, so that the gain curves of the detectors substantially coincide. To further illustrate the principles behind gain and offset mapping, reference is made to our published US Patent Application US 2011/0164139 A1.
Cameras of the IR type have traditionally made use of a plurality of fixed integration times to cover the camera's dynamic range. Every such fixed position has been provided with its own maps. The disadvantage of fixed integration times is that deviations in the image quality are necessary in order to be able to cover a particular dynamic range, since the image quality is optimized only at a single scene temperature. Several fixed integration time positions have been introduced in order to increase the image quality, which improves the image quality at the expense of higher complexity but still does not fully cover optimal integration time positions.
An alternative to fixed integration times is to change the integration time dynamically. One disadvantage associated with this is that it involves making a shift away from the integration time which was relevant at the time when the maps were made. This shift away from the integration time at the time when the maps were made gives rise to a low-frequency unevenness of the image. An extra spatial fixed-pattern noise is generated and is visible in low-contrast scenes. A previously known solution to this has been to permit the integration time to be changed only in conjunction with the offset map being updated by a so-called NUC, on uniformity correction. In order for it to function adequately, the solution requires an NUC to be performed as soon as the scene has changed significantly, for example when panning.
For an example of the prior art where the integration time is changed, reference can be made to WO 2008/107117 A1, which describes a procedure for changing the integration time depending on the temperature of an IR sensor.